The exemplary embodiment relates to the color management arts and finds particular application in connection with a system and method for printer color modeling which compensates for the influence of temperature on sensed values by taking into account area coverage of a sensed region.
Customers have come to expect a high image quality in color documents generated on electrostatographic (xerographic) image rendering devices, such as printers and copiers. One of the elements that affect the perception of image quality is an ability to consistently produce the same quality image output on a printer from one day to another, from one week to the next, month after month. Consistency between the outputs of different printers is also expected. Color rendering devices tend to exhibit a drift in their output over time, due to normally expected operational variations, such as temperature and humidity variations, system aging, and the like. Accordingly, such devices are calibrated frequently with the objective of maintaining consistent and accurate color outputs.
In-line spectrophotometric measuring systems for sensing reflectance indicative of the colors produced by the color rendering device have been developed for automated printer calibration and a variety of other color management applications. Such spectral sensors are often positioned to measure reflectance of marked sheets of print media shortly after they have been fused. Since fusing is the final step of the xerographic process, the fuser is usually located next to the paper output of the printer. Therefore due to physical constraints and space availability, the sensor is often located close to the fuser. The print media and fused colorants (typically toners) are therefore still at a temperature above ambient when the sensor measurements take place. The fused print media may remain at an elevated temperature for several minutes, so that, even in the stacker, the sheets are still at an elevated temperature. It has been found that the colors of the “hot” print media generated from the sensor measurements differ from those obtained under ambient conditions. The difference has been attributed to thermo-chromatic material properties, i.e. the toner colors shift as a function of temperature. This limits the potential accuracy of the in-line spectrophotometer since it will never “see” the printed media under the same conditions as the customer. It is desirable to provide an estimate of the color as it would be perceived by the customer at ambient temperatures.
Recent data from operational studies of in-line spectrophotometric systems suggest that color measurement differences occur between colors, when measured at the embedded location, with respect to similar measurements of the same prints made at ambient temperature. Such color measurement differences can be responsible for significant accuracy errors between the ultimately desired output color and the actual output color. Empirically-determined error differences (deltaE, or dE*) can be computed, e.g., in a range between a measurement at 60° C. and an ambient temperature of 22° C.
Mathematical methods have been developed to correct this thermo-chromatic error. One method involves building a mathematical model, implemented in the form of a thermo-chromaticity compensation matrix that relates thermo-chromatically shifted (hot) colors to thermo-chromatically stable (cool) colors. This matrix is then applied as a signal processing function to subsequent in-line color measurements, thus producing a final spectral measurement that closely approximates the stable (cool) color.
It has now been found that a global application of the single compensation matrix to all colors which assumes a linear relationship between hot and cool colors can result in errors in some regions while providing improvements to other regions.
There remains a need for a system and method for compensating for thermochromaticity errors which overcomes these problems.